It is not possible to restore SQL Server 2008 backup on SQL Server 2005. It just doesn’t work.
Alternative is to script database objects and database data from SQL Server 2008 and then execute those scripts on 2005 version. Note that you will have to review the scripts and manually remove all object types that don’t exist in 2005 version such as Geometry for example.
Another thing you can do is to create an empty database on 2005 version and use
SQL Diff[
^] and
SQL Data Diff[
^] tools from ApexSQL (also possible using other tools such as
Red Gate[
^] ) to compare and synchronize SQL 2008 backup and newly created 2005 database. Tools will take care of all issues mentioned above automatically.